This invention relates to a process for preventing the rephosphorization of killed steel being produced in a basic electric furnace. More particularly, it relates to a process for preventing the so-called rephosphorization phenomenon in which, during the production of killed steel in a basic electric furnace, a part of the phosphorus oxide formed and removed in the oxidizing period is reduced in the subsequent deoxidizing period to increase the phosphorus content of the melt agian.
The conventional process for producing killed steel in an electric furnace generally comprises the oxidizing refining step in which the molten iron or melt placed in the furnace is decarburized and heated by blowing oxygen therethrough, the skimming step in which the slag is removed from the melt, and the deoxidizing and desulfurizing step in which such materials as slag formers, deoxidizing agents, and ferroalloys for controlling the composition are added to the melt.
In the above-described oxidizing refining step, dephosphorization takes place along with decarburization, as represented by the following equation: EQU 2P+50+nCaO.fwdarw.P.sub.2 O.sub.5 .multidot.nCaO
It is generally believed that this reaction proceeds favorably when the following conditions are satisfied.
(1) The oxygen content of the melt is high.
(2) The reaction temperature is low.
(3) The CaO content of the slag is high.
(4) The P.sub.2 O.sub.5 content of the slag is low.
It is necessary for the removal of phosphorus, therefore, to supply oxygen to the melt at a relatively low-temperature stage of the oxidizing refining step, while adding thereto lime (quick lime or limestone) and fluorite for promoting the slagging of the lime.
Especially in cases where an electric furnace is used to produce high-grade steels requiring the removal of such impurities as sulfur and oxygen, the oxidizing refining step is generally followed by a step serving for deoxidization and desulfurization, which is often referred to as the reducing period. In this period, the slag of the oxidizing period is skimmed off, and such materials as deoxidizing agents and slag formers are added to effect the reducing reactions for the removal of oxygen and sulfur. Accordingly, if a part of the slag of the oxidizing period ramins in the reducing period, the phosphorus pentoxide (P.sub.2 O.sub.5) contained therein is reduced to cause the rephosphorization phenomenon in which the phosphorus content of the melt is increased again.
On the other hand, in an electric furnace using dolomite brick for the lining, the phosphorus pentoxide (P.sub.2 O.sub.5) absorbed in the lining during the oxidizing period is reduced by reaction with the deoxidizing agents used during the reducing period, so that the phosphorus content of the melt is increased again. This also accounts for at least a part of the rephosphorization phenomenon.
The countermeasures which are currently taken for the purpose of preventing the above-described rephosphorization phenomenon are to use large amounts of lime and fluorite in the oxidizing period to minimize the phosphorus pentoxide (P.sub.2 O.sub.5) contained in the slag of the oxidizing period, and to remove the slag of the oxidizing period as completely as possible before the reducing period is initiated. However, it is very difficult to remove more than 90% of the slag present in the electric furnace, and it is impossible to remove completely the residual slag of the oxidizing period adhering to the wall surfaces of the furnace. It is inevitable, therefore, that a certain degree of rephosphorization occurs in the conventional process.
Even if the above-described preventive measures are taken, low-alloyed steels undergo such a magnitude of rephosphorization as it is illustrated in FIG. 2. Specifically, the magnitude of rephosphorization is approximately 0.004% on the average. This phenomenon constitutes the greatest difficulty encountered in the production of low-phosphorus steels.